A qjit() compatible while-loop decorator for PennyLane/Catalyst.

This decorator provides a functional version of the traditional while loop, similar to jax.lax.while_loop. That is, any variables that are modified across iterations need to be provided as inputs and outputs to the loop body function:

  • Input arguments contain the value of a variable at the start of an iteration

  • Output arguments contain the value at the end of the iteration. The outputs are then fed back as inputs to the next iteration.

The final iteration values are also returned from the transformed function.

This form of control flow can also be called from the Python interpreter without needing to use qjit().

The semantics of while_loop are given by the following Python pseudo-code:

def while_loop(cond_fun, body_fun, *args):
    while cond_fun(*args):
        args = body_fn(*args)
    return args

cond_fn (Callable) – the condition function in the while loop


A wrapper around the while-loop function.

Return type



TypeError – Invalid return type of the condition expression.


dev = qml.device("lightning.qubit", wires=1)

def circuit(x: float):

    @while_loop(lambda x: x < 2.0)
    def loop_rx(x):
        # perform some work and update (some of) the arguments
        qml.RX(x, wires=0)
        return x ** 2

    # apply the while loop
    final_x = loop_rx(x)

    return qml.expval(qml.PauliZ(0)), final_x
>>> circuit(1.6)
[array(-0.02919952), array(2.56)]